1. Field of the Invention
The invention relates to a transformer, more particularly to a center-tapped transformer.
2. Description of the Related Art
Most electronic apparatus include a transformer as a core component to satisfy power transformation requirements. A transformer has an inherent leakage inductance. In particular, some magnetic lines of force generated when electricity is supplied to a primary winding do not pass through a secondary winding and thus do not generate corresponding electric current in the secondary winding. The leakage inductance is a measure of inductance of such magnetic lines of force (also called leakage flux).
In general, the leakage inductance of a transformer should be kept as small as possible. However, in some applications, the transformer is required to have a certain level of leakage inductance, such as when the leakage inductance is employed as a resonance inductance, or when the leakage inductance of a common-mode inductor is employed as a differential-mode inductance, etc.
FIG. 1 is a sectional diagram of a conventional center-tapped transformer 100, which includes a tubular spool 102, a primary winding 104, a first secondary winding 106, a second secondary winding 108, a first isolating unit 110, a second isolating unit 112, and an iron core (not shown). The spool 102 is formed with a hollow portion 114 for extension of the iron core therethrough. The primary winding 104 is wound on the spool 102. The first secondary winding 106 is wound around the primary winding 104 and is spaced apart therefrom by the ring-shaped first isolating unit 110. The second secondary winding 108 is wound around the first secondary winding 106 and is spaced apart therefrom by the ring-shaped second isolating unit 112.
FIG. 2a is a schematic diagram of an asymmetric half-bridge LLC circuit including the transformer 100, wherein (Lm) is the excitation inductance of the transformer 100 and (L1) is the leakage inductance of the primary winding 104. When a sinusoidal current (Ii) (such as the waveform 101 in FIG. 2d) is inputted into the transformer 100 at a node 15 of the circuit, the circuit will output a rectified current (Io) (such as the waveform 103 in FIG. 2d). FIGS. 2b and 2c show two different working states of the asymmetric half-bridge LLC circuit, respectively. During a positive half-cycle of the waveform of the input current (Ii), a diode (D1) conducts, a diode (D2) is cutoff, and the primary winding induces a leakage inductance (Ls1). On the other hand, during a negative half-cycle of the waveform of the input current (Ii), the diode (D2) conducts, the diode (D1) is cutoff, and the primary winding induces a leakage inductance (Ls2). In theory, the values of the leakage inductances (Ls1) and (Ls2) should be close to each other in order for the circuit to work more efficiently and to reduce power loss.
Since the leakage inductance (L1) of the primary winding 104 of the transformer 100 will vary with the change in the input current (Ii), there is a relatively large difference between the values of the leakage inductances (Ls1) and (Ls2), which in turn results in non-uniform amplitude of the output current (Io), as evident from the waveform 103 in FIG. 2d. Due to the high and low peak values of the output current (Io), the circuit experiences larger power loss, thereby restricting applications of the transformer 100 and circuits employing the same.
FIG. 3 is a sectional diagram of another conventional center-tapped transformer 200, which includes a tubular spool 202, a primary winding 204, a first secondary winding 206, a second secondary winding 208, a first isolating unit 212, a second isolating unit 210, and an iron core (not shown). The spool 202 is formed with a hollow portion 214 for extension of the iron core therethrough. The primary winding 204 is wound on an upper section of the spool 202. The first secondary winding 206 is wound on a lower section of the spool 202 and is spaced apart from the primary winding 204 by the ring-shaped first isolating unit 212. The second secondary winding 208 is wound around the first secondary winding 206 and is spaced apart therefrom by the ring-shaped second isolating unit 210.
Compared to the transformer 100 of FIG. 1, the leakage inductance of the primary winding 204 of the transformer 200 is maintained at a certain level for different circuit working states, and the insulation distance between the primary winding 204 and the first and second secondary winding units 206, 208 has a positive effect on safety specifications. Nevertheless, the leakage inductance of the transformer 200 and circuits employing the same is relatively large, which restricts applications of the same.
It is apparent from the foregoing that the conventional center-tapped transformers 100, 200 either have non-uniform leakage inductance or a rather large leakage inductance, which results in large circuit power loss and restricts applications of the same.